1. Field of the Invention
The present invention relates to a portable electric cutting machine, and more particularly to a portable electric cutting machine provided with an air-discharge mechanism for discharging air from a housing into a saw blade housing section of a saw cover.
2. Related Art
When cutting a cutting object, such as timber, by using a portable electric cutting machine (hereinafter referred to as “portable electric circular saw”), a marking line is drawn on the cutting object and then the object is cut along the marking line for the purpose of improving the cutting accuracy. More specifically, a guide piece for indicating the edge position of the saw blade is disposed at the front end of a base so that the guide piece locates on an imaginary line extended in a cutting direction from the saw blade. A user can cut the cutting object while directly confirming the positional relationship between the blade edge of the saw blade and the marking line on the cutting object. Alternatively, the user can cut the cutting object while indirectly confirming the positional relationship between the blade edge of the saw blade and the marking line by aligning the guide piece with the marking line.
However, when the bottom surface of the base slides on the cutting object, the marked line may be blurred or erased due to friction between the bottom surface of the base and the upper surface of the cutting objects, disabling the user to confirm the positional relationship between the blade edge and the marking line. Also, if ink of the marking line transfers onto the bottom surface when sliding, then the ink may get back to the cutting object so that the cutting object gets dirty.
Further, if a large volume of saw dust produced during the cutting operation piles on the cutting object, the marking line becomes invisible. In this case, the user can no longer confirm the positional relationship between the blade edge of the saw blade and the marking line, resulting in degradation of cutting accuracy and decreasing in cutting efficiency.
Saw dust accumulates on the cutting object when, for example, the saw dust produced in the cutting operation flies up and then falls down on the cutting object. Therefore, it is conceivable to use a dust collector for collecting saw dust generated during cutting operation, thereby preventing saw dust from flying up during the cutting operation. However, dust can be produced during, not only cutting operations, but also other operations using various other tools. In this case, even if such a dust collector is used during cutting operations, dust may accumulate on the cutting object to hide the marking line. In this case, a user needs to wipe off or blow off the dust on the cutting object before starting a cutting operation so that the user can see the marking line. This reduces the efficiency of the cutting operation. Also, the user needs to stop the cutting operation to remove saw dust which has been generated and piled on the cutting object during the cutting operation. This reduces both the cutting accuracy and the efficiency of the cutting operation.
Japanese Utility Model registration No. 2607480 discloses a conventional portable electric circular saw, which includes a saw blade, a motor that drives the saw blade, a housing that houses the motor, a saw cover attached to the housing, a base linked to the housing, and a fan. The saw cover has a saw blade housing section that houses the upper half of the outer periphery of the saw blade. The base has a bottom surface that slides on the cutting object. The fan is driven by the motor for generating an air current (hereinafter referred to as “fanned air”) to cool the motor. A wall of the saw cover that is positioned between the fan and the saw blade housing section is formed with fanned air discharge ports for discharging the fanned air into the saw blade housing section. The fanned air discharge ports are defined by a plurality of division walls.
Each division wall has a surface extending in parallel with a rotary shaft of the fan and in a radial direction of the rotary shaft. Thus, as passing through the fanned air discharge ports, the fanned air is directed to flow in a direction parallel to the rotary shaft of the fan and then discharged into the saw blade housing section. The fanned air discharged into the saw blade housing section in this manner collides with the lateral surface of the saw blade housed in the saw blade housing section and the lateral surface of a safety cover, which is rotatably disposed so that the saw blade housing section can house the safety cover during cutting operations. Then, the fanned air diffuses along the lateral surfaces of the saw blade and the safety cover. The diffused fanned air is then discharged to the outside of the saw cover through either between the inner wall of the saw cover and the saw blade or between the inner wall of the saw cover and the safety cover. Since the fanned air collides with the lateral surfaces of the saw blade and the safety cover substantially at a right angle, the collided fanned air diffuses substantially uniformly across wide area of the surfaces. The fanned air may be discharged from the rear side of the saw cover in the cutting direction due to air current, which is produced by rotation of the saw blade in the saw blade housing section.
The fan is a centrifugal fan. Fanned air generated by the centrifugal fan flows outwardly in radial directions of the centrifugal fan, swirls along the inner wall of the housing, and then is discharged into the saw blade housing section through the fanned air discharge ports. The flow rate and the amount of the fanned air discharged through the fanned air discharge ports are higher in areas close to the rotary axis of the fan and lower in areas remote from the rotary axis with respect to the radial directions of the fan. This arises a problem of generation of noise in radially remote areas. This problem can be overcome by downsizing the fan or reducing the rotation speed of the fan. However, this results in reducing the flow rate of the fanned air, thereby degrading the cooling performance of the fan.
Japanese Utility Model Application-Publication No. SHO-55-154101 discloses a portable electric circular saw that includes a cylindrical guide pipe disposed at the outside of a saw cover along a housing. The guide pipe is in communication with fanned air discharge ports. Fanned air discharged from the fanned air discharge ports is guided through the guide pipe to the top surface of a cutting object and blows off saw dust from the cutting object. In this manner, the saw dust and other dust are prevented from accumulating on the cutting object.
However, the guide pipe becomes an obstacle when confirming the positional relationship between the marking line and the blade edge of the saw blade, reducing the visibility of the marking line. Additionally, the assembling efficiency falls and the cost rises because of the increased number of the components.
When the cutting operation is performed, for example, in a closed area with limited space or in a room with painted walls still wet, blowing the fanned air to disperse the saw dust may not be preferable.